Chiral molecular recognition in the thermodynamics of spreading and transition for racemic and enantiomeric stearoyltyrosine films

Abstract

Equilibrium spreading thermodynamics, surface pressure vs area isotherms, and surface shear viscosities were measured for ionized monolayer films of the title compounds on an aqueous buffer subphase at pH 6.86. When allowed to equilibrate with their bulk crystalline phases (equilibrium spreading pressures), the racemic and enantiomeric films spread to significantly different surface pressures and displayed clearly defined transitions in their equilibrium spreading pressure vs temperature-phase diagrams at 30.3 and 26.3 OC, respectively. A thermodynamic analysis of the results demonstrates large differences between the racemic and enantiomeric films in terms of their changes in entropy and internal energy as they undergo the transition. In sharp contrast, racemic and enantiomeric films, spread from solution in the absence of the bulk phase, demonstrate only scant differences in their packing arrangements as reflected by surface pressure vs area isotherms taken in the same temperature range. Surface shear viscosities indicate that both spread films may be characterized as fluid. Taken together with previous results, these data indicate that short-range forces govern enantiomeric discrimination in fluid monolayer films and that the mechanism of detectable chiral molecular recognition between enantiomers lies in the transition to a tightly packed, crystalline surface state.